(1) Field of the Invention
The present invention relates to a general purpose objective lens and an optical pickup device usable in a recording and reproducing apparatus which can correspond to an optical recording medium, for example, a compact disc (CD: including CD such as CD-R and the like), a digital versatile disc (DVD), a Blu-ray Disc™, an HDDVD and the like.
More particularly, the present invention relates to an objective lens and an optical pickup device recording and reproducing at a high numerical aperture such as the Blu-ray Disc™ or the like.
(2) Description of Related Art
Conventionally, as the objective lens of the optical pickup for the CD and the DVD, a plastic single lens is used mainly. In a standard specification of the Blu-ray Disc™ corresponding to a next generation DVD (a digital versatile disc), the NA is defined as 0.85, and a glass or plastic single lens is generally used.
However, if it is intended to achieve the high NA by the single plastic lens mentioned above, a spherical aberration is significantly generated in a beam spot caused by a temperature change, for example, a change amount of the spherical aberration comes to about 80 mλrms in 15° C. change in the plastic objective lens having a focal distance 1.41 mm, and goes beyond 70 mλrms corresponding to Marechal's critical value. Since the NA is about between 0.60 and 0.67 in the conventional DVD, the spherical aberration amount generated by the temperature change is comparatively small, and it is not necessary to cancel the spherical aberration. However, in the lens having the high NA equal to or higher than 0.81, the spherical aberration becomes in proportion to the fourth power of the NA, and the spherical aberration amount generated by the temperature change becomes large. Accordingly, it is necessary to cancel the spherical aberration generated by the temperature change.
In this case, if the glass lens is used, the spherical aberration caused by the temperature change comes to about some tenth of the plastic lens and does not appear, however, there is a problem that a productivity is inferior with a high cost. Accordingly, it is desired to apply the plastic objective lens to the Blu-ray Disc™.
Then, there has been conventionally proposed a method of canceling a spherical aberration of an objective lens generated at a time of a temperature change, by moving a collimate lens in an optical pickup optical system, as described in patent document 1 (JP-A-2008-4169).
The Blu-ray Disc™ has two kinds of discs including a single-layer one and a two-layer one. The single-layer disc has a single recording layer and a thickness of a transparent board thereof is 0.100 mm, and the two-layer disc has two recording layers and thicknesses of respective transparent boards thereof are defined to 0.075 mm and 0.100 mm by a standard specification. In the case of the two-layer disc, a great spherical aberration about 0.25 λrms is generated by a difference 0.025 mm of the board thicknesses, however, the spherical aberration is compensated by moving a collimator lens or a beam expander so as to change a state of a light beam incoming to the objective lens to a parallel light incoming radiation or a divergent light incoming from a convergent light incoming, thereby recording or reproducing each of the layers.
Since a so-called object distance as seen from the objective lens is changed by compensating the difference between the spherical aberrations in the two-layer disc and the spherical aberration of the plastic objective lens at a time of the temperature change, by means of the optical system mentioned above, a field angle property (an image height property) of the objective lens is changed. In general, in the objective lens corresponding to the two-layer disc, it is designed at a thickness 0.875 mm corresponding to an intermediate thickness of the two-layer disc. However, if so, the field angle property under a certain condition does not come to a desirable value, and a lens tilt property mentioned below does not come to a desirable value. This matter is in detail described below on the basis of comparative examples in FIGS. 4 to 7.
In the pickup optical system, in order to prevent the aberration of the pickup optical system from fluctuating even at a time when the objective lens moves in a vertical direction to an optical axis at a time of a tracing servo, the incoming light to the objective lens is set to the parallel light, that is, the object distance for the objective lens is set to ∞, particularly in the case that the numerical aperture (NA) is large. Accordingly, in the case of the objective lens for the Blu-ray Disc™ corresponding to the two-layer disc having the thicknesses 0.075 mm and 0.100 mm of the transparent boards, there can be considered to set a ∞ objective lens at 0.0875 mm corresponding to the intermediate disc thickness. By using the lens mentioned above, for example, it is possible to set the incoming light to the objective lens to the convergent light in the case that the thickness of the transparent board is 0.075 mm, set the incoming light to the objective lens to the divergent light in the case that the thickness of the transparent board is 0.100 mm, and make shift degrees of the convergent light and the divergent light from the parallel light approximately identical. Therefore, it is possible to record or reproduce in a good wave aberration state in each of the recording layers.
FIG. 1 shows an example of a typical pickup optical system using a plastic objective lens for a Blu-ray Disc™ corresponding to a two-layer disc having thicknesses 0.075 mm and 0.100 mm. A laser light radiated from a semiconductor laser 1 incomes to a collimator lens 2 so as to form a parallel light, and incomes to an objective lens 4 via an aperture limiting member 3, and the laser light focuses on an information recording surface (not shown) of an optical disc 5 corresponding to the Blu-ray Disc™. In this case, a reflecting member such as a reflecting mirror or the like and a prism are omitted in this drawing, and beams of light are expressed by being linearized.
A semiconductor laser 8 serving as a laser light source emits a laser light in a bluish violet band pass (405 nm) in accordance with a standard specification of the Blu-ray Disc™. The collimator lens 2 is installed at a position at which the laser light emitted from the semiconductor laser 1 and existing in an optical path of the laser light goes forward as a divergent light, and corresponds to a plus lens refracting the laser light in such a manner as to be converted into the parallel light from the divergent light in the case that the collimator lens exists at a standard position. The collimator lens 2 is held by a collimator lens actuator (not shown) in such a manner as to be moved forward and backward and be regulated its position only in a direction of an optical axis (that is, a direction of a beam axis of the laser light emitted from the semiconductor laser 1). The objective lens 4 is a focusing lens (a plus lens) focusing the laser light transmitting the collimate lens 2 on a recording layer of the optical disc 5. The objective lens 7 is a plastic single lens formed in accordance with a standard specification of the Blu-ray Disc™. Accordingly, a numerical aperture (NA) thereof is 0.85.
In this case, a description will be given of a case of the objective lens of the parallel light incoming (∞ system) in which the thickness of the transparent board is 0.0875 mm. A focal distance is 1.411765 mm in a state of 25° C. Data of the objective lens and a calculation result of the wave aberration are shown in FIG. 4. In this case, meanings of respective symbols in FIG. 4 are as follows.
SA3: third order spherical aberration
SA5: fifth order spherical aberration
SA7: seventh order spherical aberration
SA9: ninth order spherical aberration
AS3: third order astigmatism
COMA3: third order comma aberration.
Further, the field angle property in the case that the angle of field is 0.3 degree expresses a property at a time when the light having an angle 0.3 degree with respect to the optical axis incomes to the objective lens, the lens tilt property in the case that the angle of lens tilt is 0.3 degree expresses the wave aberration property at a time when the objective lens is tilted at 0.3 degree, and the disc tilt property in the case that the angle of the disc tilt is 0.3 degree expresses the wave aberration property at a time when the objective lens is tilted at 0.3 degree. In this case, a refractive index of the objective lens is 1.53 as shown in FIG. 4, and this is a refractive index in the case of 25° C. at 405 nm. Further, a temperature property of a change amount of the refractive index is −0.00011/° C. In general, the temperature property of the change amount of the refractive index of the plastic for lens is about −0.0001/° C. to −0.00012/° C. Further, the refractive index of the transparent board of the disc is 1.62, and the temperature property of the change amount of the refractive index is −0.00011/° C.
In the objective lens shown in FIG. 4, under this state, that is, under a state in which the thickness of the transparent board of the disc is 0.0875 mm at 25° C., the COMA3 at a time when the lens tilt is 0.3 degree is −0.0286347 λRMS, and the COMA3 at a time when the disc tilt is 0.3 degree is −0.0285179 λRMS, they are approximately equal to each other. In this case, since the COMA3 in the case that the angle of field is 0.3 degree is −0.0000035 λRMS and is almost close to zero, and is designed in a state of keeping a so-called sine condition, the COMA3 at a time of the disc tilt is equal to the COMA3 at a time of the lens tilt. In this state, if the objective lens tilt is carried out, for example, at a time of recording or reproducing the tilted disc or the surface shift disc, that is, if the objective lens is set to a tilted state with respect to the optical axis by using the tilt actuator or the like, the COMA3 in the disc tilt and the COMA3 in the lens tilt cancel with each other so as to achieve a good wave aberration state as a whole of the good pickup optical system.
For example, in the case that the COMA3 of the lens tilt is generated only one half of the COMA3 in the disc tilt, the COMA3 is not canceled until the lens tilt is carried out at 1.0 degree in the case that the disc tilt is 0.5 degree. Accordingly, it is necessary to secure a dynamic range of the lens tilt sufficiently large, and it is necessary to take into consideration a structure and a dimension of the optical pickup such that no problem is generated by carrying out a great amount of lens tilt. Therefore, there is generated a defect that the optical pickup is enlarged in size and a device is required in the tilt actuator mechanism.
FIG. 5 shows the lens data and the wave aberration property at a time of recording or reproducing the thicker disc of the two-layer disc or recording or reproducing the single-layer disc by using the objective lens, that is, at a time when the thickness of the transparent board is 0.100 mm, in the state of 25° C. In this case, the object distance for the objective lens comes to a state of 338.9928 mm. In other words, as shown in FIG. 3, the collimator lens 2 moves close to the laser side in comparison with the state in FIG. 1, by the collimate lens actuator constituted by a stepping motor and a piezo element, and the laser light incomes to the objective lens in a state of the divergent light.
FIG. 6 shows the lens data and the wave aberration property at a time of recording or reproducing the thinner disc of the two-layer disc, that is, at a time when the thickness of the transparent board is 0.075 mm, by using the objective lens in the state of 25° C. in the same manner. In this case, the object distance for the objective lens comes to a state of −323.9406 mm. In other words, as shown in FIG. 2, the collimator lens 2 moves close to the disc side in comparison with the state in FIG. 1, by the collimate lens actuator, and the laser light incomes to the objective lens in a state of the convergent light.
Next, FIG. 7 shows the lens data and the wave aberration property at a time of recording or reproducing the thicker disc of the two-layer disc or recording or reproducing the single-layer disc by using the objective lens, that is, at a time when the thickness of the transparent board is 0.100 mm, in a state of 55° C. The object distance for the objective lens comes to a state of 163.3295 mm, that is, the divergent light incomes. In this case, as shown in FIG. 7, the COMA3 at a time when the angle of field is 0.3 degree is 0.024932 λRMS, the COMA3 at a time when the lens tilt is 0.3 degree is −0.0077500 λRMS, and the COMA3 at a time when the disc tilt is 0.3 degree is 0.0325419 λRMS. In other words, there is established an expression (COMA3 at a time when angle of field is 0.3 degree)=(COMA3 at a time when lens tilt is 0.3 degree)+(COMA3 at a time when disc tilt is 0.3 degree), under a precision of 1% in the COMA3.
In this case, the COMA3 at a time when the lens tilt is 0.3 degree is only 0.238 times the COMA3 at a time when the disc tilt is 0.3 degree. In other words, when it is intended to cancel the COMA3 generated in the case that the disc tilt is 0.3 degree by the lens tilt, it is necessary to carry out the lens tilt at 1.260 degree. This matter causes such a defect that it is necessary to secure a space so as to carry out a lot of lens tilt, and the mechanism of the tilt actuator for the lens tilt becomes complicated. In other words, this problem is generated because a sensitivity of the COMA3 at a time of the lens tilt becomes lower, that is, 0.238 times a sensitivity of the COMA3 at a time of the disc tilt.